When Francis Barber, born a slave on a Jamaican sugar plantation, arrived in England to begin a 32-year association with Samuel Johnson, uncertainty over his status epitomised the confusion among 18th-century judges about who should be classed a servant and who a slave.

As Michael Bundock, a lawyer at Stephenson Harwood, stresses in this excellent book, ‘unlike the position in Jamaica, there was no formal regulation of slavery, no slave code laid down in acts of parliament’. Johnson, who got to know Barber through his friend Dr Richard Bathurst and took him on as a household servant, was adamant that Barber was not a slave.

Before the advent of an organised anti-slavery movement and the Slavery Abolition Act of 1833, Johnson lambasted slave-owning colonists as ‘a Race of Mortals whom I suppose no other Man wishes to resemble’.

Yet Barber, who went on to inherit money and goods on trust when Johnson died, continued to come under scrutiny, especially in Sir John Hawkins’ Life of Samuel Johnson.

The great legal minds of the age wrestled with the issue of slavery in courtrooms. Butts v Penny and Pearne v Lisle addressed whether ‘the practice was justifiable because those who were enslaved were not Christians, but heathens’ and whether baptism, and its implicit conversion to Christianity, therefore entitled a slave to be set free.

Author: Michael Bundock

Publisher: Yale University Press (£20)

But as Bundock points out, ‘many colonial legislatures solved the problem by passing laws explicitly declaring that a slave who converted to Christianity was not thereby freed’. So when Barber was baptised – henceforth no longer known as ‘Quashey’ – he might have reflected on the words of attorney-general Sir Philip Yorke and solicitor-general Sir Charles Talbot that ‘baptism doth not bestow freedom on [a slave]’.

In 1772, the foundations of slavery began to shake. Lord chief justice Mansfield – ‘by common consent the greatest judge of the eighteenth century’ – ruled in Somerset that slavery is ‘so odious that nothing can be suffered to support it, but positive law’ (statute), and that a master could not forcibly send a man abroad to be sold.

Bundock delves deep into what motivated Johnson – the famous compiler of the English dictionary and ‘tall and muscular though ungainly, and [towering] over most people’ – to keep Barber on. He concludes that his motivation may have derived from a Christian sense of duty, especially in providing an education.

Nicholas Goodman is a sub-editor at the Law Society Gazette